Conventionally, a switching regulator employing a pulse width control system which operates by the on-off operation of switching elements has been used mainly for the constant voltage supply apparatus. The switching regulator is useful for miniaturization and is lightweight due to its high efficiency, but the existence of its switching time causes defects in that its switching loss, useless radiation noise and conduction noise, are larger. Therefore, the regulator is restricted in its service range so that, when used especially as the power source for acoustic apparatus, noise suppression countermeasures, such as filters of large attenuation inserted into the input and output units are a completely air-tight shield provided, must be used, thereby creating a problem of high manufacturing cost or low reliability.
A means of solving the above problem has been proposed, which uses a series resonant type DC-DC converter comprising a converter-transformer and a resonance capacitor connected in series thereto as shown in FIG. 1 so that switching elements are alternately on and off to obtain the predetermined output energy through the transformer. In detail, in FIG. 1, reference numerals 1 and 2 designated direct-current power sources, 3 and 4 designate switching elements, such as transistors or thyristors, 7 designates a resonance capacitor, 5a designates the primary winding of converter-transformer 5, connected in series with the resonance capacitor 7, 5b designates the secondary winding of converter-transformer 5, whose output is connected to a smoothing capacitor 9 through a rectifying diode 8, and 10 designates a load. The switching elements 3 and 4 are alternately switched so that when the switching element 3 is on and that 4 is off, a current of a sine wave form flows in a loop of DC power source 1.fwdarw.switching element 3.fwdarw.the primary winding 5a at converter-transformer 5.fwdarw.resonance capacitor 7.fwdarw.DC power source 1. On the other hand, when the switching element 3 is off and that 4 is on, a current of a sine wave form flows in a loop of DC power source 2.fwdarw.resonance capacitor 7.fwdarw.the primary winding 5a.fwdarw.switching element 4.fwdarw.DC power source 2. A cycle period of the current is equal to 2.pi..sqroot.C.sub.7 L.sub.5a decided by capacitance C.sub.7 of resonance capacitor 7 and effective inductance L.sub.5a of converter-transformer 5, the operational wave form being shown in FIG. 2. In FIG. 2, (a) and (b) show timing charts of switching elements 3 and 4 and (c) shows a current wave form corresponding to the timing shown in (a) and (b). As clearly understood from FIG. 2, since the current is zero when the switching elements 3 and 4 are alternately switched, the switching losses remarkably decrease, thereby not only expecting high efficiency but also causing a decrease in useless radiation noise and conduction noise.
The series resonant type DC-DC converter in FIG. 1, however, is difficult to stabilize against a wide variation of input and load, whereby the problem of how to control the output voltage so as to be in a stable condition still remains unsolved.
Conventionally, a series resonant type DC-DC converter utilizing the series resonance circuit comprising a resonance capacitor and a resonance coil, has been proposed in order to solve the above problem. Such a series resonant type DC-DC converter operates the series resonance circuit to cause a current of a sine wave form to flow during the conduction of a switching element so that the current and voltage intercross at the zero point when the switching elements are on and off, thereby having a characteristic of remarkably reducing the switching loss and useless radiation. In the conventional series resonant type DC-DC converter, however, it has been difficult to stabilize the output DC voltage without hindering the above characteristic with respect to the input and load fluctuation.